Multi-source DOM inputs dictate net extra carbon mineralization in eutrophic lakes via non-linear co-metabolism effects

Microbially mediated decomposition of dissolved organic matter (DOM) is a key process in the carbon cycle of inland lakes. Exogenous DOM inputs can alter the decomposition of resident lake-water DOM through co-metabolic effects (CE), yet how DOM source and addition level jointly regulate this process remains poorly understood, especially in eutrophic lakes. Here, we selected Lake Taihu, a representative hypereutrophic lake, and conducted incubation experiments by separately adding DOM derived from rainwater, leaves of aquatic plants, terrestrial litter, and treated sewage effluent to algal-rich lake water at matched carbon-addition levels. Water chemistry and DOM optical properties were used to identify factors associated with source- and concentration-dependent CE. DOM from aquatic plants and rainwater, which exhibited relatively strong aromatic and humified characteristics, induced positive CE. In contrast, terrestrial litter-derived DOM produced a weaker positive CE, while sewage-derived DOM, characterized by stronger fresh autochthonous and microbial signatures and lower average molecular weight, induced negative CE. The magnitude of CE varied nonlinearly with addition level, peaking at intermediate concentrations. CE was closely related to DOM aromaticity, humification, fresh autochthonous characteristics, and nitrogen conditions. A source-resolved scenario analysis combining estimated DOM-C inputs with source-specific CE indicated that the negative contribution of sewage-derived DOM could offset much of the positive contribution from natural sources, although the combined effect remained a potential net increase in carbon mineralization. These findings demonstrate that DOM from different environmental sources can induce CE in opposing directions in bloom-affected Lake Taihu water and that CE magnitude depends nonlinearly on loading level.